Piezoelectric transducer



June 2, 1953 W. L. CHERRY,VJR

PIEZOELECTRIC TRANSDUCER Filed May 17. 1949 R UV 5 m C H Lm A m m H 1 I000 so'oo 50'00 FREQUENCY (CYCLES PER SECOND) @mmame Patented June 2, 1953 PIEZOELECTRIO TRANSDUCER Walter L. Cherry, Jr., Northbrook, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Application May 17, 1949, Serial No. 93,700

6 Claims.

This invention relates to piezo-electric transducers, and more particularly to such transducers as disclosed and claimed in the copending application of Walter L. Cherry, Jr., Serial No. 770,163, filed August 22, 1947, for Piezo-Electric Transducers, now U. S. Patent 2,538,554 dated January 16, 1951, and that of Robert Adler, Serial No. 793,892, filed December 26, 1947, for Piezo-Electric Transducers and Methods for Producing Same, now U. S. Patent 2,54.0,412 issued February 6, 1951, both of which applications are assigned to the same assignee as the present application.

The above-identified Cherry application discloses and claims a process for producing a piezoelectric effect in a polycrystalline aggregate comprising barium titanate or barium strontium titanate. Briefly, the process comprises subjecting the aggregate to a unidirectional electric polarizing field and maintaining the field until saturation of the piezo-electric effect. Upon removal of the field thereafter, the aggregate retains a remanent piezo-electric effect of substantial magnitude.

The above-identified Adler application broadly discloses and claims a piezo-electric polycrystalline aggregate transducer in which the direction of the piezo-electric axis varies at a non-uniform rate from portion to portion. As a preferred embodiment, the application discloses a piezo-electric polycrystalline aggregate transducer in longitudinally adjacent portions of which the direction of the piezo-electric axis is substantially opposite. Such a transducer may be produced by a process of reverse alternate polarization, as disclosed in the Adler application, and is characterized by a substantially greater capacity than a simpl polarized aggregate body of similar dimensions; at the same time, the electromechanical coupling coefficient of the transducer is maintained at the same order of magnitude of that of a simply polarized piece.

A particular embodiment shown and described in the Adler application comprises a bimorphic composite element including a pair of juxtaposed polycrystalline aggregate bodies having a plurality of spaced conductive strips or electrodes disposed on the outer surfaces of the element. In order to provide a pair of output terminals, alternate strips or electrodes are interconnected. In practice, the electrode structure assumes the appearance of a pair of inter-meshing combs, formed of conductive paint and fired onto the surfaces of the ceramic bodies. In order to utilize the electrical output obtained from a transducer of this sort, it is necessary to provide output leads respectively soldered or otherwise connected to the conductive paint output terminals on the surfaces of the elements. It has been found that a transducer so constructed must be handled with extreme care to avoid rupturing the joint between the output leads and the conductive paint terminal strips.

It is therefore, an important object of the present invention to provide an improved bimorphic polycrystalline aggregate transducer of the type described above which does not require as extreme care in handling as do previous transducers of this type.

There is a further disadvantage inherent in the use of electrodes of the inter-meshing comb type. With such an arrangement, the individual electrodes extend to within a relatively short distance of the shorting strip constituting the opposite output terminal. Since the materials used are characterized by an extremely high dielectric constant, of the order of 1200 or more, the inter-meshing comb arrangement results in an undesirable shunt capacity between the output terminals and in an undesirable piezo-electric component in a transverse direction. Both of these undesirable effects act to reduce the useful output voltage. On the other hand, if a large distance is left between the individual strips and the opposite output terminal, so as to avoid the undesirable shunt capacity, a large portion of the aggregate remains unused for providing useful electrical output, and the efficiency of mechano-electrical conversion is low.

It is a further object of the invention, therefore, to provide an improved bimorphic piezoelectric polycrystalline aggregate transducer, of the reverse alternate polarization type, in which the mechano-electrical conversion efficiency is high, and. at the same time, in which the previously encountered undesirable shunt capacity and transverse piezo-electric component efiects are effectively minimized.

A piezo-electric transducer constructed in accordance with the present invention comprises a composite bimorphic element including a pair of juxtaposed polycrystalline aggregate bodies and having permanent piezo-electric properties with the direction of the piezo-electric axis difierent in adjacent portions in each of the bodies. Conductive strip terminals or electrodes are disposed between such adjacent portions on the outer surfaces of the composite element. A plurality of terminal leads are supported between the bodies and individually electrically connected with respective ones of the strip terminals, and a pair of conductors respectively connect alternate terminal leads to provide a pairof output terminals.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing, in the several fi'g'ur'es of which likereference numerals' -indicate like elements, and in which:

Figure 1 is a plan view, partiallly cut away, of

a hearing aid microphone constructed in accordance with the present invention,

Figure 2 is a sectional view takena'longthe line 22 of Figure 1,

Figure 3 is a perspective viewsot t'he transducer element used in the microphone of Figure 1, and

Figure 4 is a graphical representation of the frequency response characteristic ofthe microphone of Figure 1.

The hearing aid microphone shown in plan wiewlinFigure 1- comprises aconductive-case H1 in which is supported a piezo-electric transducer '5 I i .constructed' in a: manner hereinafter to be escribeddn detail. Transducer i i is supported t ea'ch end-i'byblocks i2- and l3 ofpl-asticizecl cellulose nitrate or other suitable firrnbut fiex- -ible material. -Transducer-li is connected-with its output across a resistor- M, which resistor may omprise the. grid resistor in the input circuit of -hearing ald-amplifier' (not shown). One of the output term-inals I 5* is connected to case L i ii, and the other output terminal 58 is connected to a :lead' [-1 of-resistoni 4. Lead i'l extends through an :insulating 'bushing- 18 to provide an-output 'lead' fol connection inan electrical circuit. The

" otlrer lead' la -extends through case 1 w and is soldered thereto. diaphragm 2d; partof *which -has beerr out away to "show 'the internal contructionofrthe: micr'ophone,- is peripherallysupported"on' a flange of case :18 as by cement or 1 the likeg a'nd is secured at its center to a loading 1n 2! whichwis -"centrally secured to transducer 1 "by: means of cem'ent or the like.

With' referenceto Figure 2 ,transducer H- comrises a p'air of polycrystalline aggregate bodies a'nd' z3 which:are ceme-nted or otherwise-seurely fastened-together by means of a'--layer=24 cementor thelike. BodiesdZ-and 23 are'contruCt'ed of at polycrystalline "aggregate and have ateipolarization: process-described and claimed 'ev'iously mentioned Adler application.

r. and 23 thereby 1: possess permanent izo -electricproperties with the direction of -the piezo' elec'tric airis" substantially 'POPDOSltB in: ionitudinally adjacent' portions in-eacl1 of the r T-he'-direction of" the piezo-electric axis eachport'i'on is indicated "schematically by the k rro ws in-Figure 2, these arrows being placed adjacent bodies 22 and 23 for the'purpose of 'avoidingcdnfusion in the" drawing- Bodies 22 and- 213 are so 'iarr'an'ged" that the direction 'of' the iezo elec't'ric axis in' each portion of body 22 is 'pposite 'to -the 'direction' of the piezo' electric axis in 'the adjacent portion of body 23. Thus,

ndividu'ally beensubjectedto the -reverse alter- 'o'dies 22' -"and"=23 form-a composite bimorphic 6 ductive strips 25-3 i. Alternate terminal leads 35, 3?, and 39 are interconnected by means of a conductor 40 to form one output terminal, and the remaining terminal leads 36 and 38 are inter- 5 connected by a second conductor M to provide a second output terminal.

-With this arrangement, when the composite element is loaded at" its center'bydiaphragm fixed to loading pin 2!, those portions of body 1022 which are placed in tension are immediately "adjacent corresponding portions of body 23 which ""are"placed' in compression. Since the direction of --*Jh8-IJl8ZO-6l86tli0 axis in each portion of body 22: is opposite to-the direction of the piezo-electric 15 axis in thefia'diacent portion of body 23, the output voltages generated by bodies 22 and 23 and appearing between output terminals 40 and M, rei'nforoe each other. For example, when the portion of body 22 between electrodes 26 and 2'! 20.- is .placed-in tension, the portion of body 23 between electrodes-ShandflZ is placed in compres- -sion.- Since the-directions of the piezoeelectric axesin these 'two portions are opposite, the .electricaloutput between-electrodes and'Zl-due 25 =to-.the tensile stress. on body 22 isof the same polarity as the voltage-appearing betweenelectrodes 3 hand 32 dueto the compressive'stress on body -23.

- Asbest seen inFigure 1,bodies 22 and ZS are so-wider. at the-center than-atthe ends. Thisdimensional relationship is .utilized to approach conditions of unifior-m stressin thetransducer when the element is loaded. --Thus, bodies 22 =-and 23, supported at. the ends by: means of supporting blocks i 2 and tdandloaded in the-center by means .of loading spin. :2 l [comprise uniform -strength beama and voptimum dynamic condi *tions are obtained.

.Inaccordance Withone feature of the present 40 invention, the centralwconductive strips or.elec- -trodes Hand 32 are connected to case ili iby means of terminal 31 and conductorlil. .Because the-central-point at which lcading'pin 2i is-affixed: is thus maintained at-the samepoten- -tiaL -as case [0; diaphragm -20 1 may beconduc- -tivelyaffixed :to loadingpin-2i to provide effec- -tive= electrostatic shieldingfor transducer II.

Ihe-construction of-the transducer element is 'shown irr detailxin-Figure 3. Inaccordance with 5 :theinventiomthe terminal leads 35-39 are sup- Jported between. bodies 22.-and23, and adjacent terminal :leads "are electrically connected to the respective conductive strips onthe' outer surfaces of theicomposite element at opposite edges there- 0 of. 'Ihese electrical connections may convenientlybe omade by daubs 142- 16. of conductive spa-int bridging the :gaps between the ends of the --respective terminal leads 3 5 -39 and the respectiveconductive strips2529. 'With this arrangement, flexure of terminal leads-35-39-.which occurs-in ordinary handling of the unit-is pre- :vented from rupturing the electrical connection between the terminal leads and theelectrodes on'the outer-"surfaces oftheelement-since-the terminal leads are firmly supported between .the component bodies.

2 Merelyby way of illustration-,sand in no'sense iby 'way'of limitation, itmayube desirable todescribe thes process of: manufacturing a unit such as tliat'shown irrFigure 3. Bodies 22 and 23may each be:constructed of a ceramic composition com prising substantially 98% ofbarium titanate and substantially2%- of stannic'oxide boundJby a ceramic binder; as "disclosed and claimediin 7 the *copending -.,appli-cation 20f Antonio;.R,. Rodfir-inch at the center.

for Ceramic Materials, now U. S. Patent 2,533,140 issued December 5, 195-0, and assigned to the present assignee. Bodies 22 and 23 may individually be inch long, and the transverse dimensions may be .-inch at each end and The thickness of each of the bodies may be .00'75-inch.

The conductive strips on the outer surfaces of the bodies may be formed of silver or other conductive paint applied thereto by means of a silk screen process and subsequently fired to the ceramic surface. In order to avoid diificulties arising from the paint spreading duringthe silk screen process, the individual strips are made ductive strips may be equally spaced and may individually have a width of .020-inch.

The bodies 22 and 23 so prepared are then individually subjected to the reverse alternate polarization process described and claimed in the above-mentioned Adler application; briefly, asteady electric field is applied to the bodies, the direction of the field being opposite between adjacent pairs of electrodes. By way of example, an electric field of 20,000 volts per centimeter may be applied for 20 minutes.

After removing the electric field, the bodies are juxtaposed so that the directions of the piezoelectric axes in immediately adjacent portions thereof are everywhere opposite as explained in connection with Figure 2.

Terminal leads 3539 are interposed between the bodies in parallel relation with the conductive strips, and the bodies are firmly united by cement or the like. Terminal leads 35-39 may be of molybdenum and may individually have a thickness of .001-inch and a width of .OlO-inch. Daubs 42-45 of conductive paint are applied to connect terminal leads 3539 to the respective conductive strips on the outer surfaces of the composite element.

In order to provide a pair of output terminals, terminal leads 35, 37, and 39 are connected to conductor 40 which may be a nickel strip, and. terminal leads 36 and 38 are connected to another conductor 4| similar to conductor 40.

A unit so formed is characterized by a capacity of about 40 mi-cro-microfarads and an electromechanical coupling coeificient of about 25%.

The structure of Figure 3 effectively minimizes the undesirable shunt capacity encountered in similar transducers utilizing eletcrode patterns in the form of a pair of inter-meshing combs while providing improved mechano-electrical conversion efiiciency. This desirable result is due to the fact that the output terminals 40 and M are removed from the ends of the conductive strips 25-29 and are separated'from the ceramic bodies 22 and 23 by air having a dielectric constant much smaller than that of the polycrystallin-e aggregate.

Figure 4 is a graphical representation of the frequency response characteristic of a hearing aid microphone constructed in accordance with Figures 1-3. In Figure 4, the microphone voltage output is plotted in decibels as a function of the sound frequency. The ordinates of the curve of Figure 4 are referred to a zero-decibel reference of one volt from a pressure applied at the loading point of one dyne per square centimeter. The response characteristic is quite similar to that obtained with a conventional crystal microphone now used in commercial hearing aids, being characterized by a resonant rise at a freshorter than the width of the bodies. The conquency of about 3,000 cycles per second and having a useful range from about 200 cycles per second to about 4,000 cycles per second.

Throughout the specification, and in the appended claims, the eifect induced by the polarization of a polycrystalline aggregate is termed a piezo-electric effect. It is recognized that there exists a controversy over the proper terminology for this effect, and some eminent authorities refer to the effect as an electrostrictive one. It is to be clearly understood that, in the sense used in this application, the term piezo-electric is to be construed as descriptive of the property of a material of developing electrical voltage in response to mechanical stress.

The present invention provides a novel type of piezo-lectric transducer which is inexpensive, rugged, durable, and readily adaptable to mass production techniques. A microphone comprising a transducer constructed in accordance withthe invention affords a frequency response characteristic comparable with that obtained with a conventional hearing aid microphone embodying a Rochelle salt crystal, but at a much lower cost.

While a particular embodiment of the present invention has been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A piezo-electric transducer comprising: a composite bimorphic element including a pair of juxtaposed polycrystalline aggregiate bodies and having permanent piezo-electric properties with the direction of the piezo-electric axis different in adjacent portions of each of said bodies; conductive strip terminals disposed on the outer surfaces of said composite element between said adjacent portions; a plurality of terminal leads supported between said bodies for the entire width thereof and individually electrically connected with respective ones of said strip terminals; and a pair of conductors respectively connecting alternate ones of said terminal leads to provide a pair of output terminals.

2. A piezo-electric transducer comprising: a composite bimorphic element including a pair of juxtaposed polycrystalline aggregate bodies and having permanent piezo-electric properties with the direction of the piezo-electric axis different in longitudinally adjacent portions of each of said bodies; spaced conductive strip terminals disposed on the outer surfaces of said composite element between said longitudinally adjacent portions; a plurality of terminal leads supported between said bodies for the entire width thereof in parallel relation with and electrically connected to respective ones of said strip terminals; and a pair of conductors respectively connecting alternate ones of said terminal leads to provide a pair of output terminals.

3. A piezo-electric transducer comprising: a composite bimorphic element including a pair of juxtaposed thin substantially diamond-shaped polycrystalline aggregate bodies and having permanent piezo-electric properties with the direction of the piezo-electric axis substantially opposite in longitudinally adjacent portions of each of said bodies; spaced conductive strip terminals disposed transversely of said axis on the outer surfaces of said composite element between said longitudinally adjacent portions; a plurality of terminal leads: supported between-said bodies 1 or the: entirevwidth thereof in parallel relationwith respective ones ofsaid stripterminals; adjacent ones-10f. said terminal leadsbeing electrically connecteditd said-respective strip terminals at oppositetedges. ofsaid composite element; and'a pair of;1conductorsrespectively connecting the free ends-of alternate ones of said terminal leads to provide a pairof output terminals.

rte-VA:piezo-electr-ic transducer comprising: a composite biniorphic. element including-apair of juxtaposed uniform-estrength beams .of polycrystalline aggregateihaving permanent piezo-electric properties with the direction of the. pieZo-electric axis substantially opposite inlongitudinally adjacentportiens of each cit said beams; equally spaced-conductive strip terminalsrdisposed transverselyof said on the outer surfacesof said composite-element between said longitudinally adjacent portions; and a plurality of terminal leads supported between said-beams for the eratire width thereof inparallel relation with respectiveones: ofsaid strip terminals, adjacent onesoisaidterminal leadsbeing electrically con nected to said respective strip terminalslat opposits edges. of said composite element.

1.5. 'Aimicrophone ioruse inhearing aidsand the likecomprisingz. av conductive case constituting one output. terminal; apiezo-electric. transducer supported in said. case and comprising a com posite bimorphic element including a pair of juxtaposed polycrystalline aggregate bodies having permanent piezo-electric properties with the di rectionof the piezo-electric axis substantially opposite in longitudinally. adjacent portions of each 'inparallel relation with and individually electrically connected to respective ones of said strip terminals; means connecting alternate ones of said" terminal leads to said case thereby electrically connecting alternate ones of said strip terniinals'to said case; an electrically conductive resilient diaphragm. peripherally supported by 8 said casewand electrically and :mecnanically fixed tdsaid element at a pointon .oneof saidalternate strip terminals; and. meansinsulated from-:said case for interconnecting theremainder. of said terminaltlea-ds to provide a 1 second output. :tertminali 6 1A microphone for use in hearing aids and the like comprising: a case constitutinga grounded output terminal; a pieZo-eleotric transducer supported. insaid case, and comprising a composite bimorphic element including a pair of juxtaposed polycrystalline'aggregate uniform-strength beams having permanent piezo-electric properties with the direction of the pieZo-electric axis substantially opposite in longitudinally adjacent portions of. each ofilsaid beams, equally spaced conductive strip terminals disposed transversely of said axis;:on thev outer surfaces of said com posite' element between said longitudinally adjacent portions, and a plurality of'terminal leads supported between said beams in parallel relation with, and individually electrically connected torespective ones of said strip terminals; means grounding alterna-teones of said terminal leads totsaid case to ground the conductive strip terminals connected to said alternateleads; an electrically conductive resilient diaphragm. peripherally supported by said case and electrically and mechanically fixed to said element at a point-on one of said-grounded-strip terminals; and means insulated from said'case for interconnecting the remainder of said terminal leads to provide a second output terminal.

WALTER L. CHERRY, JR.

References Cited .in the me of this. patent UNITED STATES PA'I'ENTS Number Name Date 2,105,010 Sawyer Jan. 11, 1938 2,242,756 Pope May 20,1941 2,479,926 Gravley Aug. 23, 1949 2,479,987 Williams Aug. 23, '1949 2,484,950" Jaffe Oct. 18, 1949 "2,515,446 Gravley July 18, 1950 254:0,194 Ellett Feb. 6, 1951 

